iOS: The iPhone has had a mobile-formatted Google Translate webapp for a while, but as of today they have a new native app with even more features, including speak-to-translate, offline storage, and the ability to hear your translations pronounced out loud.
The app's basic functionality works just like Google Translate does online: you can type in any phrase and get it translated into more than 50 languages. For half of those languages, Google Translate can even speak the translation back to you, so you get the pronunciation down pat. You can also save your favourite phrases for later viewing offline.
Probably the coolest part of the app, though, is the speech recognition. By hitting the microphone button on the app's front page, you can input phrases from 15 languages and get them translated, which is great if you don't quite know the spelling of that phrase you heard. Hit the link to check it out.
Google Translate is a free download for iOS 3 or later, and in all iOS-supported languages.